Closed superconductive magnet with homogeneous imaging volume

ABSTRACT

A closed magnetic resonance imaging (MRI) magnet has a single superconductive coil assembly including a toroidal-shaped coil housing containing a pair of superconductive main coils and at least one additional superconductive main coil. A pair of generally annular-shaped permanent magnet arrays is spaced radially inward and apart from the superconductive main coils. The permanent magnet arrays allow the design of a shorter MRI magnet because the permanent magnet arrays overcome the gross magnetic field distortions in the imaging volume of the magnet (created by removing some of the additional longitudinally-outermost superconductive main coils, otherwise used in the magnet, to make the magnet shorter) to produce a magnetic field of high uniformity within the imaging volume.

BACKGROUND OF THE INVENTION

The present invention relates generally to a superconductive magnet(such as, but not limited to, a helium-cooled and/or cryocooler-cooledsuperconductive magnet) used to generate a high magnetic field as partof a magnetic resonance imaging (MRI) system, and more particularly tosuch a magnet having a closed design and having a homogeneous (i.e.,uniform) magnetic field within its imaging volume.

MRI systems employing superconductive or other type magnets are used invarious fields such as medical diagnostics. Known superconductivemagnets include liquid-helium cooled and cryocooler-cooledsuperconductive magnets. Typically, for a helium-cooled magnet, thesuperconductive coil assembly includes a superconductive main coil whichis at least partially immersed in liquid helium contained in a heliumdewar which is surrounded by a dual thermal shield which is surroundedby a vacuum enclosure. In a conventional cryocooler-cooIed magnet, thesuperconductive main coil is surrounded by a thermal shield which issurrounded by a vacuum enclosure, and the cryocooler coldhead isexternally mounted to the vacuum enclosure with the coldhead's firststage in thermal contact with the thermal shield and with the coldhead'ssecond stage in thermal contact with the superconductive main coil.Nb-Ti superconductive coils typically operate at a temperature ofgenerally 4 Kelvin, and Nb-Sn superconductive coils typically operate ata temperature of generally 10 Kelvin.

Known superconductive magnet designs include closed magnets and openmagnets. Closed magnets typically have a single, tubular-shapedsuperconductive coil assembly having a bore. The superconductive coilassembly includes several radially-aligned and longitudinallyspaced-apart superconductive main coils each carrying a large, identicalelectric current in the same direction. The superconductive main coilsare thus designed to create a magnetic field of high uniformity within aspherical imaging volume centered within the magnet's bore where theobject to be imaged is placed. Although the magnet is so designed tohave a highly uniform magnetic field within the imaging volume,manufacturing tolerances in the magnet and magnetic field disturbancescaused by the environment at the field site of the magnet usuallyrequire that the magnet be corrected at the field site for such minorirregularities in the magnetic field. Typically, the magnet is shimmedat the field site by using pieces of iron, or, for Nb-Ti superconductivemagnets cooled by liquid helium, by using numerous Nb-Ti superconductivecorrection coils. The correction coils are placed within thesuperconductive coil assembly radially near and radially inward of themain coils. Each correction coil carries a different, but low, electriccurrent in any required direction including a direction opposite to thedirection of the electric current carried in the main coils. Closed MRImagnets tend to have a relatively long axial (i.e., longitudinal) lengthto accommodate the number of main superconductive coils needed toachieve a homogeneous imaging volume which, especially in the case ofwhole-body magnets, tends to create claustrophobic feelings in patients.

Open magnets typically employ two spaced-apart superconductive coilassemblies with the space between the assemblies allowing for access bymedical personnel for surgery or other medical procedures during MRIimaging. The patient may be positioned in that space or also in the boreof the toroidal-shaped coil assemblies. The open space helps the patientovercome any feelings of claustrophobia that may be experienced in aclosed magnet design. The literature is largely silent on howsuperconductive open magnets can be made to have a magnetic field ofhigh uniformity within the imaging volume when the creation of the openspace between the superconductive coil assemblies grossly distorts themagnetic field creating a magnetic field of low uniformity within theimaging volume. Such magnetic field distortion is well beyond that whichcan be overcome by using known magnet shimming technology. Also, suchopen magnets are more expensive than closed magnets for the samestrength magnetic field within the imaging volume.

What is needed is a closed MRI magnet which is designed to have arelatively short axial (i.e., longitudinal) length to overcomeclaustrophobic feelings of patients and to provide at least some patientaccess by physicians and which is designed to have a highly uniformmagnetic field within its imaging volume to provide for sharp medicalimages.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a closed superconductive MRImagnet.

The closed MRI magnet of the invention has a single superconductive coilassembly including a generally toroidal-shaped coil housing surroundinga bore and having a generally longitudinally-extending axis. The singlesuperconductive coil assembly also includes a pair oflongitudinally-spaced-apart, generally identical, and generallyannular-shaped superconductive main coils each generally coaxiallyaligned with the axis, each having a generally identical firstmagnetic-field direction within the bore generally parallel to the axis,each located within the coil housing, and each having alongitudinally-outermost portion. The single superconductive coilassembly additionally includes a pair of longitudinally-spaced-apart,generally identical, and generally annular-shaped permanent magnetarrays each generally coaxially aligned with the axis, each radiallypositioned inward and apart from the pair of superconductive main coils,and each longitudinally positioned completely between thelongitudinally-outermost portions of the pair of superconductive maincoils. The single superconductive coil assembly further includes atleast one generally annular-shaped additional superconductive main coilgenerally coaxially aligned with the axis, having a magnetic-fielddirection within the bore generally identical to the firstmagnetic-field direction, and positioned within the coil housing andlongitudinally between the pair of permanent magnet arrays. In apreferred embodiment, the pair of permanent magnet arrays each islongitudinally positioned a generally identical distance from a planewhich is oriented perpendicular to the axis and which is locatedlongitudinally midway between the pair of superconductive main coils. Inan exemplary embodiment, the coil housing has spaced-apartlongitudinally-outermost ends, and the bore has a radius which increasesas one moves longitudinally outward from the pair of permanent magnetarrays to the longitudinally-outermost ends of the coil housing.

Several benefits and advantages are derived from the invention. WithApplicant's closed MRI magnet design, the overall axial (i.e.,longitudinal) length of, for example, a whole-body magnet may beshortened by using magnetic field analysis to choose the permanentmagnet arrays to overcome the gross magnetic field distortions withinthe imaging volume (created by removing some, or all, of the additionallongitudinally-outermost superconductive main coils removed to shortenthe magnet) to produce a magnetic field of high uniformity within theimaging volume. Applicant's highly uniform magnetic field permits highquality MRI imaging. Applicant's shorter closed magnet design eliminatesor reduces any claustrophobic feelings of patients. Applicant's designof a coil housing whose bore opens wider at its ends further reduces anyfeelings of claustrophobia.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate two preferred embodiments of thepresent invention wherein:

FIG. 1 is a schematic cross-sectional side-elevational view of a firstpreferred embodiment of the closed MRI magnet of the invention havinginternal permanent magnet arrays;

FIG. 2 is a view, as in FIG. 1, but of a second preferred embodiment ofthe closed MRI magnet of the invention having external permanent magnetarrays; and

FIG. 3 is a schematic end view of the MRI magnet of FIG. 2 taken alongthe lines 3--3 of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like numerals represent likeelements throughout, FIG. 1 shows a first preferred embodiment of theclosed magnetic-resonance-imaging (MRI) magnet 110 of the presentinvention. The magnet 110 has a single superconductive coil assembly112. The single superconductive coil assembly 112 includes a generallytoroidal-shaped coil housing 114 surrounding a bore 116 and having agenerally longitudinally-extending axis 118. The coil housing 114includes a first generally-circumferential outside surface 120 facinggenerally towards the axis 118 and a second generally-circumferentialoutside surface 122 radially spaced apart from the first circumferentialoutside surface 120 and facing generally away from the axis 118. Thecoil housing 114 also includes spaced-apart longitudinally-outermostends 124 and 126. Each of the longitudinally-outermost ends 124 and 126preferably is a generally-annular outside surface. Typically, the coilhousing 114 defines a vacuum enclosure.

The single superconductive coil assembly 112 also includes a pair oflongitudinally-spaced-apart, generally identical, and generallyannular-shaped superconductive main coils 128 and 130 and furtherincludes at least one generally annular-shaped additionalsuperconductive main coil (only one additional pair 132 and 134 of whichis shown in FIG. 1) which typically is not generally identical to thepair of superconductive main coils 128 and 130. The additionalsuperconductive main coils are needed to achieve a high magnetic fieldstrength, within the magnet's imaging volume, without exceeding thecritical current density of the superconductor being used in the coils,as is known to those skilled in the art. Each of the pair ofsuperconductive main coils 128 and 130 (and each of the additionalsuperconductive main coils such as the one additional pair 132 and 134)is conventionally supported on a coil form (not shown in the figures).Each of the pair of superconductive main coils 128 and 130 (and each ofthe additional superconductive main coils) is generally coaxiallyaligned with the axis 118, is disposed within the coil housing 114, andcarries a generally identical main electric current in a same firstelectric-current direction. The first electric-current direction isdefined to be either a clockwise or a counterclockwise circumferentialdirection about the axis 118 with any slight longitudinal component ofcurrent direction being ignored. Hence, each of the pair ofsuperconductive main coils 128 and 130 (and each of the additionalsuperconductive main coils) has a generally identical firstmagnetic-field direction within the bore 116 which is generally parallelto the axis 118. Preferably, the pair of superconductive main coils 128and 130 is a longitudinally-outermost (i.e.,longitudinally-furthest-apart) pair of superconductive main coils. Thesuperconductor used in each of the superconductive main coils 128, 130,132, or 134 typically would be a superconductive wire or superconductivetape wound such that each superconductive main coil 128, 130, 132, or134 has a longitudinal extension and a radial extension (i.e., radialthickness) far greater than the corresponding dimensions of thesuperconductive wire or superconductive tape. Each of the pair ofsuperconductive main coils 128 and 130 has a longitudinally-outermostportion 136 and 138.

The single superconductive coil assembly 112 additionally includes apair of longitudinally-spaced-apart, generally identical, and generallyannular-shaped permanent magnet arrays 140 and 142. It is noted that theat least one additional superconductive main coil 132 and 134 isdisposed longitudinally between the pair of permanent magnet arrays 140and 142. Each of the pair of permanent magnet arrays 140 and 142 isgenerally coaxially aligned with the axis 118, radially disposed inwardand apart from the pair of superconductive main coils 128 and 130, andlongitudinally disposed completely between the longitudinally-outermostportions 136 and 138 of the pair of superconductive main coils 128 and130. Each of the pair of permanent magnet arrays 140 and 142 may be anarray of circumferentially-abutting permanent-magnet ring segments or anarray of spaced-apart, circumferentially-adjacent permanent-magnet ringsegments wherein the length of the arc of any circumferential spacebetween circumferentially-adjacent permanent-magnet ring segments isalways smaller than the arc length of any permanent-magnet ring segmentmaking up the array. In the first preferred embodiment, as shown in FIG.1, the pair of permanent magnet arrays 140 and 142 is disposed withinthe coil housing 114 proximate the first circumferential outside surface120. In an exemplary construction, the pair of permanent magnet arrays140 and 142 consists essentially of (and preferably consists of) a pairof ferrite, samarium-cobalt, or neodymium-iron arrays. Preferably, thepair of permanent magnet arrays 140 and 142 each is longitudinallydisposed a generally identical distance from a plane 144 which isoriented perpendicular to the axis 118 and which is disposedlongitudinally midway between the pair of superconductive main coils 128and 130.

It is preferred that the longitudinally-outermost ends 124 and 126 ofthe coil housing 114 are longitudinally disposed a generally identicaldistance from the plane 144. It is noted that suchlongitudinally-outermost ends 124 and 126 desirably aregenerally-annular outside surfaces facing generally away from (andpreferably aligned parallel with) the plane 144. The firstcircumferential outside surface 120 defines the radial extent of thebore 116 of the coil housing 114. In an exemplary construction, the bore116 has a radius which increases as one moves longitudinally outwardfrom the pair of permanent magnet arrays 140 and 142 to thelongitudinally-outermost ends 124 and 126 of the coil housing 114. Thisreduces claustrophobic feelings in patients who are placed in the bore116 of whole-body MRI magnets. Preferably, the radius of the bore 116 isgenerally constant as one moves longitudinally across the pair ofpermanent magnet arrays 140 and 142, and the radius of the bore 116 isgenerally constant as one moves longitudinally between the pair ofpermanent magnet arrays 140 and 142. In FIG. 1, the radius of the bore116 is generally identical longitudinally across and between the pair ofpermanent magnet arrays 140 and 142. "Additionally, the bore has aradius which linearly increases as one moves longitudinally outward fromproximate the pair of permanent magnet arrays to proximate thelongitudinally-outermost ends of the coil housing."

The pair of superconductive main coils 128 and 130 (together with theadditional main coils such as the additional pair of superconductivemain coils 132 and 134) typically produce, as designed by the artisan, agenerally spherical, generally ellipsoidal (as shown in FIG. 1),generally cylindrical, or other-shaped imaging volume 146 (shown as adotted image in FIG. 1) typically centered generally at the intersectionof the plane 144 and the axis 118. A closed MRI magnet which is designedto have a relatively short axial (i.e., longitudinal) length to overcomeclaustrophobic feelings of patients must remove somelongitudinally-outermost superconductive main coils to achieve its shortlength. The effect of the removed coils is to distort the uniformity ofthe magnetic field of the imaging volume 146. As one moveslongitudinally through the imaging volume 146, the magnitude of themagnetic field decreases with decreasing distance from the center of theimaging volume 146 because of the missing longitudinally-outermostsuperconductive main coils which were removed to shorten the length ofthe magnet 110. The effect of the pair of permanent magnet arrays 140and 142 is to lower the magnitude of the magnetic field toward thelongitudinal edges of the imaging volume 146 in line with the lowermagnitude at the center. The pair of permanent magnet arrays 140 and 142is designed, using the principles of the present invention, previouslydisclosed herein, together with conventional magnetic field analysis, asis within the skill of the artisan, to produce a highly homogeneousmagnetic field within the imaging volume 146 for improved MRI imaging.In a preferred enablement, the pair of permanent magnet arrays 140 and142 each has a magnetic field direction within the bore 116 generallyopposite to the first magnetic-field direction within the bore 116 ofeach of the pair of superconductive main coils 128 and 130. Preferably,superconductive shielding coils 148 and 150 also are employed whichcarry a shielding electric current in a direction opposite to the firstelectric-current direction, as can be appreciated by those skilled inthe art.

Referring again to the drawings, FIGS. 2-3 show a second preferredembodiment of the closed magnetic resonance imaging (MRI) magnet 210 ofthe present invention. Magnet 210 is similar to magnet 110 of the firstpreferred embodiment of the invention, with differences as hereinafternoted. The bore 216 has a generally constant radius as one moveslongitudinally between the longitudinally-outermost ends 224 and 226 ofthe coil housing 214. The pair of superconductive main coils 228 and 230(together with the additional main coils such as the additional pair ofsuperconductive main coils 232 and 234) have been designed to produce agenerally spherical imaging volume 246, as is within the skill of theartisan. The pair of permanent magnet arrays 240 and 242 is disposedoutside (and not within) the coil housing 214 in the bore 216.Preferably, as shown in FIG. 2, the pair of permanent magnet arrays 240and 242 is attached to the coil housing 214. It is noted that in apreferred construction the pair of generally identical permanent magnetarrays 240 and 242 each comprises circumferentially-abuttingpermanent-magnet ring segments wherein the circumferentially-abuttingring-segment nature of permanent magnet array 242 is shown in FIG. 3. Itis pointed out that permanent magnet array 240 is generally identical topermanent magnet array 242. In magnet 210, as in magnet 110, it ispreferred to employ superconductive shielding coils 248 and 250.

It is noted that magnet cooling mechanisms, together with any requiredthermal shields, do not form a part of the present invention and havebeen omitted from the figures. Any cryogenic cooling mechanism, such as,but not limited to, liquid helium (or other cryogenic fluid) coolingand/or cryocooler cooling may be employed in combination with thepresent invention.

The foregoing description of several preferred embodiments of theinvention has been presented for purposes of illustration. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be defined by the claims appended hereto.

We claim:
 1. A closed magnetic-resonance-imaging magnet comprising asingle superconductive coil assembly including:a) a generallytoroidal-shaped coil housing surrounding a bore and having a generallylongitudinally-extending axis; b) a pair of longitudinally-spaced-apart,generally identical, and generally annular-shaped superconductive maincoils each generally coaxially aligned with said axis, each having agenerally identical first magnetic-field direction within said boregenerally parallel to the axis, each disposed within said coil housing,and each having a longitudinally-outermost portion; c) a pair oflongitudinally-spaced-apart, generally identical, and generallyannular-shaped permanent magnet arrays each generally coaxially alignedwith said axis, each radially disposed inward and apart from said pairof superconductive main coils, and each longitudinally disposedcompletely between said longitudinally-outermost portions of said pairof superconductive main coils; and d) at least one generallyannular-shaped additional superconductive main coil generally coaxiallyaligned with said axis, having a magnetic-field direction within saidbore generally identical to said first magnetic-field direction, anddisposed within said coil housing and longitudinally between said pairof permanent magnet arrays, wherein said pair of permanent magnet arrayseach is longitudinally disposed a generally identical distance from aplane which is oriented perpendicular to said axis and which is disposedlongitudinally midway between said pair of superconductive main coils,wherein said pair of permanent magnet arrays each has a magnetic fielddirection within said bore generally opposite to said firstmagnetic-field direction, wherein said pair of permanent magnet arraysis disposed within said coil housing, wherein said radius of said boreis generally constant as one moves longitudinally across and betweensaid pair of permanent magnet arrays, and wherein said coil housing hasspaced-apart longitudinally-outermost ends and wherein said bore has aradius which linearly increases as one moves longitudinally outward fromproximate said pair of permanent magnet arrays to proximate saidlongitudinally-outermost ends of said coil housing.
 2. The magnet ofclaim 1, wherein said pair of permanent magnet arrays each comprisescircumferentially-abutting permanent-magnet ring segments.